Short-pulse fiber-laser

ABSTRACT

A mode-locked fiber laser has a resonator including a gain-fiber, a mode-locking element, and a spectrally-selective dispersion compensating device. The resonator can be a standing-wave resonator or a traveling-wave resonator. The dispersion compensating device includes only one diffraction grating combined with a lens and a minor to provide a spatial spectral spread. The numerical aperture of the gain-fiber selects which portion of the spectral spread can oscillate in the resonator.

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to fiber lasers. The inventionrelates in particular to mode-locked fiber lasers.

DISCUSSION OF BACKGROUND ART

Stable, low-noise sources of ultra short pulses are important for a widerange of applications including ultrafast spectroscopy, multi-photonexcitation microscopy, micromachining, harmonic generation, and thepumping parametric amplifiers and oscillators. Such sources are usuallyreferred to by practitioners of the art as ultrafast lasers. The mostcommon ultrafast lasers are solid-state lasers having a crystalgain-medium, such as titanium-doped sapphire (Ti:sapphire), with a broadgain-bandwidth.

Fiber lasers offer an attractive alternative to solid-state ultrafastlasers. Such lasers provide an ability to operate in a wide range ofpulse-repetition frequency, for example, between about 10 megahertz(MHz) and 400 MHz and can be contained in a compact package. Compared tosolid-state crystal lasers, fiber lasers have high intra-cavitydispersion. This is because most of the resonator (cavity) optical pathof the laser is in glass (the fiber) and not in air. It is well knownthat the less the intra-cavity dispersion, the shorter the pulses thatcan be obtained.

There are a few methods for reducing intra-cavity dispersion in fiberlasers. One uses chirped fiber Bragg gratings (CFBG). Another uses acompressor and stretcher arrangement based on bulk diffraction gratings.Yet another method uses a photonic-bandgap fiber or other specialtyfiber with a specially designed dispersion-profile to compensatedispersion provided by a gain-fiber and intra-cavity components.

Chirped fiber Bragg gratings (CFBGs) are often used for generatingpicosecond pulses from an all-fiber cavity when fiber dispersion is notcompletely compensated. At low dispersion, however, for example lessthan 1 picosecond per nanometer (ps/nm), a CFBG typically has a lowreflectivity (<40%). Such a low reflectivity makes it difficult to usesuch gratings in mode-locked fiber-lasers wherein reflectivity requiredfor mode-locking is typically above 50%. Mode-locked fiber lasers areoften used when pulses shorter than 1 picosecond (ps) are required.

A compressor arrangement based on diffraction gratings providesadjustable dispersion, which can be tuned to exactly compensateintra-cavity dispersion. An example 10 of such a compressor arrangementis depicted in FIG. 1. A compressor 10 includes essentially identicaldiffraction gratings 12A and 12B arranged spaced apart and parallel toeach other in combination with a mirror 14 comprising a multilayerreflective coating 16 on a substrate 18.

Here, an input pulse P_(In) has a spectral bandwidth λ₃ minus λ₁, and acenter wavelength λ₂. It is assumed that, as a result of intra-cavitydispersion, shorter wavelengths such as λ₁ have been delayed more thanlonger wavelengths such as λ₃ thereby increasing the duration (length)of the pulse. Grating 12A diffracts the pulse wavelengths at differentangles with longer wavelengths such as λ₃ diffracted at a greater anglethan shorter wavelengths such as λ₁. Grating 12B directs the variouslydiffracted wavelengths along parallel paths to be incident mirror 14.Mirror 14 reflects the wavelengths back along their incident paths torecombine, on the path of the input pulse, as an output pulse P_(out).As the paths of the longer wavelengths are longer than those of theshorter wavelengths, the shorter wavelengths “catch-up” with the longerwavelengths sufficiently that output pulse has a shorter duration thatthe input pulse.

With the mirror of a “grating pair” pulse-compressor such as compressor10 used an end-mirror of a mode-locked fiber-laser pulses shorter than 1ps have been obtained. Such a compressor however has certain drawbacks.One drawback is that control of the lasing wavelength in the lasercavity with a compressor is difficult as spectral selectivity of thecompressor is very low. By way of example FIG. 2 schematicallyillustrates typical measured diffraction efficiency of a transmissiongrating such as gratings 12A and 12B. It can be seen that diffractionefficiency varies by only a few percent over a spectral (wavelength)range between about 1000 nm and 1080 nm. Accordingly, the prior artcompressor of FIG. 1 could at best be described as weakly wavelengthselective in this range, which represents about the full gain-range(emission range) of an ytterbium (Yb) doped gain-fiber.

FIG. 3 schematically illustrates a typical prior-art arrangement 20 amode-locked fiber laser including a grating-pair compressor such ascompressor 10 of FIG. 1. Laser 20 has a resonant cavity formed betweenmirror 14 of compressor 10 and a saturable Bragg reflector (SBR) 22,which provides passive mode-locking of the laser. The resonant cavity(resonator) includes an active (doped) fiber 26 and through-fibers 32and 36 of couplers 28 and 30 spliced to the active fiber. SBR 22 isformed from a saturable absorber and a Bragg reflector. One surface ofthe SBR 22 is mounted to a substrate 26 and the opposed surface isbutt-coupled to fiber 36. A lens 40 collimates radiation from the fiberportion of the resonator before the radiation enters compressor 10. Lens40 focuses radiation from the compressor back into the fiber portion ofthe resonator. A half-wave plate 42 adjusts polarization of returningradiation to maximize transmission through the gratings 12B and 12A.Coupler 30 is a wavelength-division multiplexing (WDM) coupler couplingpump radiation into the fiber portion of the resonator via a fiber 38.Coupler 28 is a fractional coupler which couples a fraction ofcirculating radiation out of the resonator, as mode-locked outputpulses, via a fiber 34.

FIG. 4 schematically illustrates the absorption (solid curve) andemission (dashed curve) spectra over a wavelength range from about 850nm to 1150 nm. It can be seen that the emission-curve has a strongnarrow peak centered at about 975 nm partially overlapping a peak in theabsorption spectrum centered at a wavelength of about 980 nm which isthe usually preferred pump-wavelength for a Yb-doped fiber. At longerwavelengths, the emission curve varies relatively strongly with a peakgain between about 1035 nm and about 1040 nm. Because of the relativelyweak spectral selectivity of compressor 10, and in the absence of anyother spectral selective device in the resonator, the gain-curve woulddominate the wavelength selection process and the resonator (pumped by980 nm radiation) would oscillate in the 1035 nm to 1040 nm peak-gainregion.

Apart from the lack of spectral selectivity another drawback of thegrating-pair compressor is that circulating radiation makes forward andreverse passes through each grating. Even given an efficiency of about95%, as indicated in FIG. 2, the four passes would introduce resonatorlosses of about 20%. While active fiber 26 has high gain and theresonator can tolerate relatively high losses, such losses detract fromoverall efficiency of a fiber laser. Yet another drawback is the highcost of the diffraction gratings, which are by far the most expensivecomponents in the laser cavity. Including two such gratings makes whatwould be a relatively inexpensive laser quite costly.

There is a need for an intra-cavity compressor arrangement which hassufficiently high spectral selectivity to determine a lasing wavelengthwithin the gain-bandwidth of a gain-fiber but does not require twodiffraction gratings. Such a compressor could enable the building oflow-cost femtosecond laser systems with controllable lasing wavelength,which could expand the range of applications for such systems.

SUMMARY OF THE INVENTION

One aspect apparatus in accordance with the present invention comprisesa laser resonator including a fiber gain-medium having a gain-bandwidth.A pump-radiation source is arranged to energize the gain-fiber forgenerating laser-radiation in the laser-resonator. A mode-lockingelement is located in the resonator for causing the laser-radiation tobe generated as mode-locked pulses, the mode-locked pulses undergoingdispersion while propagating through the gain-fiber. Awavelength-selective dispersion compensating device is located in thelaser-resonator for compensating the dispersion undergone by themode-locked pulses and selecting a center wavelength of the mode-lockedpulses within the gain-bandwidth of the gain-fiber.

In preferred embodiments of the inventive apparatus the wavelengthselective dispersion compensating device includes first and secondlenses having a transmission diffraction grating therebetween. Radiationexiting the gain fiber is collimated by the first lens diffracted into aspectrally graded fan of rays by the diffraction grating focused by thesecond lens onto a plane mirror which reflects the spectrally spreadrays back through the first lens to the diffraction grating. Thediffraction grating directs the spectrally spread rays back to the firstlens which focuses the rays back into the fiber. Spectral-selection anddispersion-compensation can be selectively varied by selectively varyingthe spacing between the grating and the second lens. Because there isonly one diffraction grating in the device, losses are significantlyreduced compared with prior-art grating pair compensation devices whichare essentially not spectrally selective.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, schematically illustrate a preferredembodiment of the present invention, and together with the generaldescription given above and the detailed description of the preferredembodiment given below, serve to explain principles of the presentinvention.

FIG. 1 schematically illustrates a prior-art pulse-compressor includingfirst and second diffraction gratings spaced part and parallel to eachother with the first grating arranged to spread a single incident beaminto diverging spectral components, the second arranged to direct thespectral components parallel to each other to be incident of a mirror,with the mirror arranged to reflect the spectral components back alongtheir incident path to be recombined by the first grating back along thepath of the incident beam.

FIG. 2 is a graph schematically illustrating absolute diffractionefficiency as a function of wavelength for a commercially availablediffraction grating as a function of wavelength in a range between 1000and 1080 nanometers.

FIG. 3 schematically illustrates a prior art mode-locked fiber laserincluding a grating-pair pulse-compressor similar to thepulse-compressor of FIG. 1 and functioning as a dispersion compensationdevice, the fiber laser having a resonant cavity formed between themirror of the pulse compressor and a saturable Bragg reflectorfunctioning as a mode-locking device.

FIG. 4 is a graph schematically illustrating absorption and emissioncross-sections as a function of wavelength in a wavelength range from850 nanometers to 1150 nanometers for a ytterbium-doped gain-fiber.

FIG. 5 schematically illustrates one preferred embodiment anintra-cavity dispersion compensating device in accordance with thepresent invention including a diffraction grating cooperative with alens and a plane mirror, with the grating and mirror each axially spacedfrom the lens by a focal length thereof such that the device is in aneutral mode providing neither dispersion nor spectral selectivity.

FIG. 6 schematically illustrates the device of FIG. 5 but with thegrating spaced from the lens by more than a focal length thereof suchthat the device provides negative dispersion and functions as a pulsecompressor and also functions in a spectrally-selective manner to limitthe spectral bandwidth of a pulse.

FIG. 6A schematically illustrates the device of FIG. 6 but with themirror tilted with respect to the lens to select a particular wavelengthof the pulse in addition to limiting the bandwidth of the pulse.

FIG. 7 schematically illustrates a preferred embodiment of alinear-resonator mode-locked fiber laser in accordance with the presentinvention, similar to the laser of FIG. 3, but wherein the grating pairpulse compressor is replaced by a spectrally-selective intra-cavitydispersion-compensating device in accordance with the present inventionin the configuration of FIG. 6.

FIG. 8 is a graph schematically illustrating measured optical spectrafor pulses of different center wavelengths generated in an example ofthe apparatus of FIG. 7 by tilting the plane mirror of the spectrallyselective dispersion compensating device as depicted in FIG. 6A.

FIG. 9 schematically illustrates a preferred embodiment of aring-resonator fiber laser in accordance with the present inventionincluding a spectrally-selective intra-cavity dispersion-compensatingdevice in accordance with the present invention in the configuration ofFIG. 6, with mode-locking provided by a saturable Bragg reflector.

DETAILED DESCRIPTION OF THE INVENTION

Continuing with reference to the drawings, wherein like components aredesignated by like reference numerals, FIG. 5 schematically illustratesone preferred embodiment an intra-cavity dispersion compensating device50 in accordance with the present invention. Device 50 includes atransmission diffraction grating 52, a positive lens 54 having anoptical axis 56, and a plane mirror 58. Mirror 58 provides one-endmirror of a linear laser-resonator including a fiber gain-medium. Aconnecting fiber 60 for the gain-medium is terminated by a ferrule 62.

A radiation pulse initially exits fiber 60 (ferrule 62) as a divergingbeam 64. Beam 64 has a numerical aperture (NA) which is determined bythe NA and core diameter of fiber 60. Diverging beam 64 is collimated bya positive lens 66 having an optical axis 68. The collimated beam isdesignated 64C in FIG. 5 and is assumed here to have a cross-sectioncentered on optical axis 68 of lens 66. Optical axis 68 intersectsoptical axis 56 of lens 54 at diffraction grating 52.

Forward propagating beam 64C has a spectral bandwidth corresponding tothe spectral bandwidth of the pulse, with all spectral componentsequally distributed over the beam cross-section. At diffraction grating52 the spectral components are spread into continuous fan of beams inthe plane of incidence of beam 64C on the diffraction grating, i.e., inthe plane of the drawing. In the drawing, only the longest and shortestwavelength beams are depicted for simplicity of illustration.

In this depiction, the point of incidence of beam 64C of grating 52 isaxially spaced from lens 54 by a distance D equal to the focal length fof the lens and mirror 58 is spaced from the lens by that same focallength. All spectral component beams are focused on the mirror to forman elongated spectrum on the mirror. All components are reflected bymirror 58 back along their incidence paths, are combined by grating 52into a return-propagating collimated beam 64C which is focused by lens66 back into fiber 60.

Those skilled in the art may recognize the particular arrangement thegrating, lens and mirror of FIG. 5 as similar to a Fourier“pulse-shaper” in which mirror 58 would be replaced by a pixilated,reflective, spatial light modulator (SLM), arranged to selectivelyattenuate or remove spectral components of a pulse. Indeed, such anarrangement could, in theory at least, provide the required selectivityfor determining the operating wavelength of a laser. However, as anobject of the present invention is to minimize cost of a laser spectralselectivity is achieved by other means.

As depicted particularly in FIG. 5, device 50 provides neitherdispersion compensation nor spectral selectivity and would never be usedin that precise configuration. Both dispersion compensation and spectralselectivity can be provided by one or more simple reconfigurations ofthe device. By way of example, the grouping of lens 54 and mirror 58,still spaced apart by focal length f, can be moved toward or away fromgrating 52, as indicated by arrows A to provide dispersion compensationtogether with some measure of spectral (primarily bandwidth) selectivityby changing the distance D. These components could be mounted on atranslation stage to create a device with selectively variabledispersion. Alternatively the combination of grating 52, lens 54 andferrule 62 could be moved, with lens 54 and mirror 58 fixed. If thegrating is moved within a focal length of the lens the sign of thedispersion will be the opposite of the case when the grating is morethan a focal length from the lens.

Center wavelength selectivity can be provided by tilting mirror 58 aboutan axis 59 perpendicular to axis 56 as indicated in FIG. 5 by arrows C.Lens 54 could be tilted in a similar manner to achieve center wavelengthselectivity. Center wavelength selectivity could also be provided bychanging the incidence point of forward-propagating collimated beam 64Con grating 52 in the incidence plane of the beam on the grating. Centerwavelength selectivity could also be provided by locating an apertureplate 55 translatable perpendicular to axis 56, as indicated by arrows Sin front of mirror 58. The width (height) of the aperture in the platecould be selected to restrict the bandwidth of the spectrum reflectedback by the mirror.

FIG. 6 schematically illustrates one configuration 50A of device 50which provides dispersion compensation and spectral bandwidthselectivity. Here the axial spacing D of grating 52 and lens 54 isgreater than focal length f of lens 54. Rays spread by grating 54 do notreturn along their incidence paths and are not recombined by thegrating. The longer wavelength components travel on a longer paththrough device 50 than the shorter wavelength components, providing thenecessary dispersion compensation.

Because the spectral components do not return on their incident paths, ahypothetical (one time) return beam of all components that is wider inthe plane of incidence (plane of the drawing) than the incident beam,and spectrally graded (spatially “chirped”) across the beam width fromthe wavelength of the shortest wavelength component to the wavelength ofthe longest wavelength component. However, only those components of thespectral gradient that fall on (overlap) the beam-path of incident bean64C (cross-hatched in FIG. 6) can be focused back into fiber 60. Othercomponents either fall outside the aperture of lens or are focused bythe lens outside of the NA of the fiber (cross-hatched beam 64) andcannot be guided by the fiber. So the bandwidth of radiation that couldoscillate in a resonator terminated by the device is determined by theconfiguration of the device and the NA of the fiber (NA of output beam64). In this example, that bandwidth has been significantly narrowed bythe device.

It should be noted, here, that in FIG. 6 rays outside the NA of thegain-fiber are depicted only to illustrate how spectral selection isachieved in the inventive device and would only exist when radiationinitially exited the fiber in a transient stage prior to lasing. Insteady-state operation, there will be no radiation at wavelengthsoutside of that band of wavelengths that return from the diffractiongrating to fall within the NA of the fiber. Accordingly, the wavelengthselection mechanism itself does not create a loss. The only significantloss will result from the double pass though the grating at less than100% efficiency per pass.

Those return-propagating components that do overlap the incident beampath will still be spectrally graded across the beam but will behomogenized by the return passage through the fiber. It should also benoted that in this configuration of the inventive device, the centerwavelength of the narrowed bandwidth would be about the same as thecenter wavelength of the bandwidth in the absence of a wavelengthselective device. However, the actual oscillating wavelength of a fiberlaser including the device could differ depending on the location of thenarrowed bandwidths on the gain-curve of the fiber.

FIG. 6A schematically illustrates another configuration 50B of device 50which provides dispersion compensation, spectral bandwidth selectivity,and center wavelength selectivity. Configuration 50B is similar toconfiguration 50A of FIG. 6 with an exception that the normal 57 tomirror 58 is inclined by a small angle a to axis 56 of lens 54. Thiscauses the spectral grading in the return direction from the grating tobe laterally displaced with respect to axis 68 of lens 66 such thatwavelengths overlapping the path of the incident beam are from the longwavelength side of the spectrum. Tilting mirror 58 in the oppositedirection would select wavelengths from the short wavelength side of thespectral gradient of the device. The selected bandwidth would be aboutthe same whatever center wavelength was selected by the tilting.

FIG. 7 schematically illustrates one preferred embodiment 80 of amode-locked fiber laser in accordance with the present invention. Laser80 is a standing-wave or linear resonator laser similar to laser 20 ofFIG. 3 with an exception that the prior-art, spectrally non-selectivegrating-pair intra-cavity dispersion compensator of laser 20 is replacedin laser 80 by a spectrally selective intra-cavity dispersioncompensator in accordance with the present invention. In laser 80, theconfiguration of the inventive compensator depicted is configuration 50Bdescribed above with reference to FIG. 6A.

FIG. 8 is a graph schematically illustrating reproductions of measuredpulse spectra from an experimental example of the apparatus of FIG. 7.In this example gain-fiber 26 is a 1% Yb-doped fiber. Fiber 32 has a NAof 0.12. Lens 66 has a focal length of 11.0 millimeters (mm). Lens 54has a focal length of 50.0 mm and is axially spaced from grating 52 by68.0 mm. This arrangement restricted spectral bandwidth to about twentynanometers. The grating is a 1250-lines-per-millimeter grating fromIbsen Photonics AS, of Farum, Denmark. The center wavelength was tunedfrom about 1020 nm (long-dashed curve) to 1045 nm (short-dashed curve)by tilting mirror 28 as illustrated in FIG. 6A. The peak gain is atabout 1035 nm at which the solid curve is tuned.

FIG. 9 schematically illustrates an embodiment 82 of a mode-lockedtraveling-wave fiber-laser in accordance with the present invention.Laser 82 includes a traveling-wave resonator 83 comprising a loop orunidirectional-propagation portion 83A and straight or bidirectionalpropagation portions 83B and 83C. The unidirectional propagation portionis formed by a dichroic beamsplitter 85, a Faraday isolator 91 whichprovides unidirectional propagation, a polarizing beamsplitter cube 90and a gain-fiber 84. Beamsplitter cube 90 connects the unidirectionalpropagation portion with the bidirectional propagation portions and thebidirectional propagation portions with each other.

The gain-fiber is optically pumped by radiation from a diode-lasersource 87. Dichroic mirror 88 is highly transmissive for thepump-radiation wavelength and highly reflective for radiation generatedby the fiber-laser. A lens 86 collimates laser-radiation from thegain-fiber and focuses the pump-radiation into the gain-fiber.Laser-radiation in the loop portion is plane-polarized in the plane ofthe drawing as indicated by arrows P_(L). Polarization in the straightportions varies according to the propagation direction and thepolarization properties of optical components through which theradiation passes.

In a round trip in resonator 83 laser-radiation from gain-fiber 86 isreflected by dichroic mirror 85 and is transmitted by polarizingbeamsplitter 90 into resonator portion 83B. The polarization planepasses through a Faraday rotator 92 and is reflected by a mirror 94 intoa spectrally-selective, intra-cavity dispersion-compensating device 50A.In accordance with the present invention, radiation is reflected backout of device 50A, traverses Faraday rotator 92 again and is reflectedby polarizing beamsplitter 90 into straight portion 83C of resonator 83.It should be noted that Faraday rotator 92 could be replaced by aquarter-wave plate to provide a 90° polarization-rotation on doublepass.

In portion 83C, radiation passes through a quarter-wave plate 96 whichcircularizes the polarization of the radiation. This radiation isfocused by a lens 98 onto a SBR 22. The radiation is reflected by theSBR; re-collimated by lens 98; plane polarized by a return passagethrough the quarter-wave plate, with a 90° rotation of the polarizationplane; and transmitted by polarizing beamsplitter cube into loop portion83A of the resonator. In loop portion 83A the radiation traverses afront surface polarizer 100 which reflects a portion, for example about20%, of the radiation as output radiation (mode-locked pulses). Theremaining radiation is focused back into gain-fiber 84 by a lens 102.

It should be noted here radiation leaving device 50A has the spatialchirp (spectral gradient) imposed by the device and retains that spatialchirp until it is focused back into the gain-fiber. Lens 102,accordingly, has the same function as lens 66 in laser 80 of FIG. 7,with the NA of the gain-fiber providing the aperture that selects thewavelength range from the spatial chirp of device 50A.

It is emphasized that although resonator 83 appears on superficialconsideration to be a ring resonator having appended branches, theresonator is in fact a single, resonator entity having the abovedescribed round-trip sequence. There is no constructive or destructiveinterference in the straight portions of the resonator, ascounter-propagating beams therein have different polarizationproperties.

In summary, fiber lasers described herein include a resonator having amode-locking element therein and an inventive wavelength-selectivedispersion compensating device including only a single diffractiongrating. Because there is only one grating losses in the device aresignificantly reduced compared with prior-art grating-pair devices whichare not spectrally selective. In the description of the lasers, themode-locking element is a saturable Bragg reflector. However, anothertype of mode-locking element based on modulating intra-cavity loss orgain, such as an acousto-optic modulator, and electro-optical modulator,or nonlinear polarization-rotation modulator may be used withoutdeparting from the sprit and scope of the present invention.

The present invention is described above in terms of a preferred andother embodiments. The invention, however, is not limited to theembodiments described and depicted. Rather, the invention is limitedonly by the claims appended hereto.

What is claimed is:
 1. Laser apparatus, comprising: a laser resonatorincluding a fiber gain-medium having a gain-bandwidth, said resonatorincluding a plane end mirror; a pump-radiation source arranged toenergize the gain-fiber for generating laser-radiation in thelaser-resonator; a mode-locking element located in the resonator forcausing the laser-radiation to be generated as mode-locked pulses, themode-locked pulses undergoing dispersion while propagating through thegain-fiber; and a wavelength-selective dispersion compensating devicefor compensating the dispersion undergone by the mode-locked pulses,said dispersion compensating device including a single diffractiongrating located within the resonator near the plane end mirror and afirst lens located between the grating and the plane end mirror, and asecond lens located between an end of the gain-fiber and the grating,and with the spacing and angular orientation of the grating, lenses andplane end mirror being fixed during operation and providing dispersioncompensation and wavelength selection, said wavelength selectionresulting at least in part because less than all the wavelengths presentin the laser radiation when the gain-fiber is initially energized aredelivered from said wavelength-selective dispersion compensating deviceback into the gain-fiber.
 2. The apparatus of claim 1, wherein thewavelength selective device selects one of a spectral bandwidth of thepulses and the center wavelength of the pulses.
 3. The apparatus ofclaim 1, wherein the laser resonator is a linear laser resonator formedbetween the plane end minor and a second end mirror.
 4. The apparatus ofclaim 1, wherein the mode-locking element is a saturable Bragg mirrorfunctioning as the second end-minor of the laser-resonator.
 5. Theapparatus of claim 4, wherein the saturable Bragg mirror is butt coupledto the gain-fiber.
 6. Laser apparatus, comprising: a linear laserresonator including a fiber gain-medium having a gain-bandwidth, thelaser resonator having a longitudinal resonator axis and beingterminated at one end thereof by a saturable Bragg reflector and at theother end thereof by a plane mirror, the saturable Bragg reflectorfunctioning as a mode-locking element; a pump-radiation source arrangedto energize the gain-fiber for generating laser-radiation in thelaser-resonator, the laser-radiation to be generated as mode-lockedpulses, the mode-locked pulses undergoing dispersion while propagatingthrough the gain-fiber; a first lens, a diffraction grating and a secondlens located in the laser resonator between gain-fiber and the planeminor, the diffraction grating being located between the first andsecond lenses; and wherein the spacing and angular orientation of thefirst and second lenses, the diffraction grating, and the plane minorare fixed during operation and selected to provide dispersion whichcompensates the dispersion undergone by the pulses propagating throughthe fiber, and to select a center wavelength of the mode-locked pulseswithin the gain-bandwidth of the gain-fiber with the center wavelengthselection resulting at least in part because less than all thewavelengths present in the laser radiation when the gain-fiber isinitially energized are delivered back into the gain-fiber after passingthrough the lenses and the grating.
 7. The apparatus of claim 6, whereinthe first lens is axially spaced from the fiber by about a focal lengthof the first lens, the second lens is axially spaced from the planemirror by about a focal length of the second lens, and the second lensis axially spaced from the grating by a distance different from thefocal length of the second lens.
 8. The apparatus of claim 7, whereinthe distance between the grating and the second lens is selectivelyvariable for varying one or more of the compensating dispersion, thecenter wavelength of the mode-locked pulses within the gain-bandwidth ofthe gain-fiber, and the spectral bandwidth of the mode-locked pulses. 9.The apparatus of claim 7, wherein the plane minor is selectivelytiltable about an axis perpendicular to the longitudinal resonator axisfor selecting a center wavelength of the mode-locked pulses within thegain-bandwidth of the gain-fiber.
 10. A pulsed fiber laser comprising: aresonator terminated by first and second end mirrors; a gain fiberlocated within the resonator; a pump source for exciting the gain fiber;a saturable absorber in the resonator arranged to mode-lock the laser toproduce laser pulses; a single transparent grating located in theresonator between an end of the gain fiber and the first end mirror; anda first lens located between the grating and the end mirror, and asecond lens located between said end of the gain fiber and the gratingand with the spacing and angular orientation of the grating, lenses andfirst end minor being fixed during operation and arranged to compensatefor dispersion induced in the laser pulses from the fiber and forwavelength selection, said wavelength selection resulting at least inpart because less than all the wavelengths present in the laserradiation when the gain-fiber is initially excited are delivered backinto the gain-fiber after passing through the lenses and the grating.11. A laser as recited in claim 10, wherein a normal to the surface ofthe first end minor is tilted with respect to the optical axis of thefirst lens.
 12. A laser as recited in claim 11, further including anaperture plate located between the first lens and the first end mirrorfor narrowing the spectral bandwidth of the laser.